The TSS Earnslaw will return to work two weeks later than planned.
Steel plating below the lower deck was found to be more corroded than expected during a dry dock survey.
The extra work was discovered when the timber decking was removed for replacement.
Real Journeys boss Richard Lauder says: “While it is disappointing the vintage steamship will now not be back in service for the school holidays, Walter Peak farm tours will still run as per usual using our launch.”
The 100-year-old ship, which was pulled out of the water at Kelvin Grove slipway late last month, is expected to return to Lake Wakatipu on Saturday, July 14.
The tourist steamer, which celebrates its centenary in October, comes out of the water every two years for a full inspection and hull survey.
Along with the additional remedial work, her exterior will be repainted, her lower deck will be re-decked, the starboard shaft will be pulled out for inspection, the engine will be overhauled, the upholstery will be redone and a crank-shaft will be taken out through a hole in the roof then repaired.